1. Field of the Invention
The present invention is directed to processes for biologically converting lignocellulosic biomass. Particularly, the invention relates to an improved method to ferment biomass material, which reduces the need for costly physical agitation, for the production of ethanol. Additionally, other useful chemical products can be produced from the biological conversion of the biomass.
2. Background Art
Lignocellulosic biomass, which is available in abundance, can be used as a feedstock for production of fuels and chemical. A variety of plant biomass resources are available as lignocellulosic materials for the production of biofuels, notably bioethanol. The major sources are (i) wood residues from paper mills, sawmills and furniture manufacturing, (ii) municipal solid wastes, (iii) agricultural residues and (iv) wood chips, and (v) energy crops.
Independent of the status and future prospects of the corn ethanol industry, ethanol production from cellulosic biomass, such as wood, grass, and agricultural residues, has attracted a great deal of attention of late. Although cellulosic ethanol is not yet produced commercially, projected features include a decisively positive fossil fuel displacement ratio, near-zero net greenhouse gas emissions, potential for substantial soil fertility and carbon sequestration benefits, and feedstocks with broad geographical diversity, expected to be widely available at a cost per unit energy (e.g. $/GJ) equal to that provided by oil were it is available at about $17/barrel.
The primary obstacle impeding the more widespread production of energy from biomass feedstocks is the general absence of low-cost technology for overcoming the recalcitrance of these materials. As outlined above, cellulosic ethanol can be produced from a wide variety of cellulosic biomass feedstocks including agricultural plant wastes (corn stover, cereal straws, sugarcane and sugarcane bagasse), plant wastes from industrial processes (sawdust, paper pulp), consumer waste and energy crops grown specifically for fuel production, such as switchgrass. Cellulosic biomass is composed of cellulose, hemicellulose and lignin, with smaller amounts of proteins, lipids (fats, waxes and oils) and ash. Roughly, two-thirds of the dry mass of cellulosic materials are present as cellulose and hemicellulose. Lignin makes up the bulk of the remaining dry mass.
The production of ethanol from biomass typically involves the breakdown or hydrolysis of lignocellulose-containing materials into disaccharides and, ultimately, monosaccharides. Biological processing cellulosic biomass aims to extract fermentable sugars from the feedstock. The sugars in cellulose and hemicellulose are locked in complex carbohydrates called polysaccharides (long chains of monosaccharides or simple sugars). Separating these complex polymeric structures into fermentable sugars is essential to the efficient and economic production of cellulosic ethanol.
A number of processing options are employed to produce fermentable sugars from cellulosic biomass. One approach utilizes acid hydrolysis to break down the complex carbohydrates into simple sugars. An alternative method, enzymatic hydrolysis, utilizes pretreatment processes to first reduce the size of the material to make it more accessible to hydrolysis. Once pretreated, enzymes are employed to convert the cellulosic biomass to fermentable sugars, which can be fermented by industrial microorganisms to produce fuel ethanol or other useful chemicals, but it is critical to use an efficient process to keep costs as low as possible.
However, cellulosic ethanol production presents a number of challenges that must be met in order to economically and efficiently produce ethanol from biomass. For example, challenges exist in the removal of solids from the production stream of cellulosic ethanol. In the biological production of alcohol from plant materials, the biomass is mixed with hot water to produce a wort, which is fermented by a microorganism. The fermented contents are then typically discharged as a slurry (“beer”) and then alcohol is removed by distillation. The remainder, after distillation, is non-fermented insoluble material known as “stillage,” and consists of a large amount of water together with the solids. Another challenge is the recalcitrance of lignocellulosic material to breakdown and the high cost of enzymes used in this conversion.
Many factors are involved in efficient bioprocessing, but the final concentration of product and good mixing are two of the most important. In order to achieve a high concentration of the fuel or chemical product, it is necessary to start with high concentrations of an initial starting material (substrate). With biomass fermentation, the use of high substrate concentrations creates problems for another key factor, mixing. Cellulosic biomass is highly fibrous, strong and water-absorbing, making it very difficult to mix at high concentrations. Mechanical mixing of cellulosic biomass requires a great deal of energy expense in the form of electricity to drive the impellers and is costly.
It would therefore be an advance in the art to achieve good mixing with less need for mechanical agitation so that high substrate concentrations can be used. The present invention describes a new way to achieve good mixing in biomass fermentation, without costly mechanical agitation of the biomass substrate, by using a flow-through reactor.